Optical attenuation compares input and output power on a logarithmic scale. When powers are in linear units, the loss in decibels is: Attenuation (dB) = 10 × log10 (Pin / Pout) If the link length L is provided, the attenuation coefficient is: Coefficient (dB/km) = Attenuation (dB) / L (km) For dBm. what a fiber run has as the loss value (measured in dB). There are no specific requirements for this document. This document is not restricted to specific software and hardware versions. If either Tx or Rx is in the -30 dBm or lower range that's usually indicative of there being no actual signal received and the transceiver is reporting. For optical fiber, testing includes fiber geometry, attenuation and bandwidth. The core diameter, cladding diameter and concentricity. Attenuation in fiber optics is the gradual loss of light signal strength as it travels through a fiber cable.